Key Concepts in Chemical Equilibria
Generic Equilibrium:
k1
aA + bB
cC + dD
k-1
At equilibrium rate forward = rate reverse
k1(AA)a(AB)b = k-1(AC)c(AD)d
AA = “activity” of compound (or element) A, AA = A[A] (for solutions). For
unit activity coefficient:
k1[A]a[B]b = k-1[C]c[D]d
Rearranging:
Keq = k1 = [C]c[D]d = [products]x
k-1
[A]a[B]b
[reactants]y
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Key Concepts in Chemical Equilibria
IMPORTANT!
1. In order for a system (reaction) to be at equilibrium, the Keq
expression MUST be satisfied!!!
2. If the Keq exp. is satisfied, the system is in equilibrium!
Golden Rules of Equilibria:
1. Keq is dimensionless and constant at a given temperature.
2. When writing equilibrium constant expressions, omit solids, pure
liquids, and solvents.
3. Always use smallest integer coefficients when balancing equations
and writing Keq.
4. When a balanced reaction is reversed, Kreverse = 1/Kforward
5. When adding reactions, Keq for the net reaction is the product of the
Keq's for the individual reactions.
6. At equilibrium, the concentrations of reactants and products MUST
satisfy the equilibrium constant expression!!
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Key Concepts in Chemical Equilibria
Using Keq to predict "direction" of reaction:
• When given a set of conditions (concentrations) ask:
1. Is the system already in equilibrium?
2. If yes, you're done! If no, what does the system need to do to
reach equilibrium?
Le Chatelier's Principle:
• Start with system in equilibrium, what happens if we perturb
equilibrium?
Autoprotolysis of water:
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
Kw=[H3O+][OH-] = 1.00x10-14
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Key Concepts in Acid-Base Equilibria
Acid/Base Strength:
• "Strength" = measure of efficiency of production of H+ (or OH-),
extent of dissociation.
• Know strong acids/bases assume everything else is weak! (H2SO4,
HCl, HNO3, HClO4, NaOH, KOH, LiOH)
Solution acidity and pH:
• Because water is amphiprotic, "pure" water will contain a small
amount of OH- and H+
• pH is a measure of “[H+]”: pH = -log[H+]
• Ka for an acid and Kb for its conjugate base are related!
• pH and pOH are related!
• ICE table approach to determining pH of weak acid/base solutions
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